Wireless local area network (WLAN), which is one of developing wireless communication technology, is a technology for laptops, mobile devices, or other terminal stations (hereinafter, STAs) to link to the interact from home, company or WLAN server using wireless radio technology.
FIG. 1 illustrates a structure of 802.11 wireless local area network (WLAN) system which is created by Institute of Electrical and Electronics Engineers (IEEE).
IEEE 802.11 WLAN system includes a plurality of components which works mutually to provide STAs portable wireless local network. Basic service set (BSS), which is a basic component in IEEE 802.11, is assemble of STAs using the same radio frequency within a specific radius. FIG. 1 takes WLAN consists of BSS 101 and BSS 111 and one distribution system (DS) 121 as an example. A first BSS 101 includes one member STA 103. A second BSS 111 includes two members, STA113 and STA 115. DS 121 is connected to both first BSS 101 and second BSS 111. Access point (hereinafter AP) 131, 133 can receive STAs and can be view as a STA. That is, AP 131, 133 can transmit data between BSS and DS.
In the present disclosure, STAs can be terminals, wireless transceivers, user devices, mobile stations, portable terminals, user terminals, receiving devices, etc. AP can be a base station (BS), a node B, an evolved node B (eNodeB) in a WLAN system or any kind of access points connected between WLAN and the internet.
STAs and Aps in WLAN system have to be synchronized. Therefore, APs periodically or non-periodically transmit beacon signals so that STAs can implement synchronization with STAs according to the received beacon signals.
STAs need to scan channels before receiving and transmitting data in WLAN system. However, STAs keep consuming power when scanning channels. As a result, the power consumption in receiving mode is higher than transmitting mode. High power consumption in receiving mode makes usage time of STA become shorter because STAs usually utilize batteries as power source. Hence, if STAs keep scanning channels for remaining in receiving mode, power is wasted even no data transmission happens within WLAN system. To solve this problem, the power management (PM) modes of STAs in WLAN system require improvement.
PM modes can be divided into active mode and power save (PS) mode. STAs usually utilize active mode to remain in wake-up mode. Transmission and reception of signal frames or channel scanning can be implemented normally in wake-up mode. STAs switch between sleep mode and wakeup mode to save power consumption. STAs do not transmit nor receive signal frame and do not scan channel either.
The longer the time when the STA remain in sleep mode is, the less power loss of the battery is, and the usage time of STA becomes longer accordingly. However, STAs cannot receive and transmit signal frames in sleep mode, thus, it is impossible for STAs to always remain in sleep mode. If reception or transmission of signal frames is required, STAs have to switch to wake-up mode from sleep mode. However, if APs require to receive or transmit signal frames to STAs, STAs will be unable to transmit signal frames required by Aps in sleep mode and unable to know the requirement of receiving signal frames transmitted from Aps. In order to know whether the reception of signal frame transmitted from APs is required or not, STAB need to switch to wake-up mode from sleep mode in specific period. The specific period can be the period of beacon signals transmitted by APs. Thus, STAs need to estimate reception time of beacon signals precisely.